1. Field of the Invention
This invention relates in general to an image forming apparatus configured to form a full-color image by superimposed colors with a one-time charge of an image bearing member.
2. Discussion of the Background
An image forming apparatus, such as a printer, a facsimile machine, a copy machine, a plotter, or a printer/facsimile/copy complex machine, is known to form an image according to the following electrophotographic process: charging an image bearing member (also referred to herein as a “photo conductor”), forming an electrostatic latent image, developing the image by providing adherence of powder (also referred to herein as “toner particles”), to create a toner image, and transferring the toner image to other medium.
Currently in an image forming apparatus using the above electrophotographic process, there are two different processes in a full-color image forming apparatus that superimposes color on color. One type is to rotate one image bearing member four times. During each rotation, a process of uniform electrostatic charge, a process of exposing the image, a process of forming an image by development with any one of color toners (Cyan, Magenta, Yellow, Black), and a process of transferring the developed image to an intermediate transfer member or recording medium so as to fit position each other, are performed. In the other type of process, four photo conductors are laterally arranged. For each photo conductor, the process of uniform electrostatic charge, the process of exposure image, the process of forming an image by development with any one of color toners (Cyan, Magenta, Yellow, Black), and the process of transferring the developed image on each photo conductor to an intermediate transfer member or a recording medium so as to fit position each other is performed.
However, the one photo conductor/four rotations type process has a problem of slow printing speed. And forming an image with the laterally arranged four photo conductor type process (tandem type process) has disadvantages of larger and complicated structure, and higher cost.
Consequently, another type of process has been designed. This type is capable of superimposing color toner on other color toner during one-rotation of one photo conductor (hereinafter referred to as a “one-photo-conductor/one-rotation-superimpose-type process”). And, although, there is a method of superimposing different color toner on a surface of the photo conductor by one-photo-conductor/four-rotations-type process also, this type of process has a problem of a slow printing rate. To distinguish herein the type of rotating one photo conductor four times and transferring every single color toner image per one-rotation, and the type of superimposing multiple color toners on the photo conductor without transferring every single toner image, the former is referred to as one-photo-conductor/four-rotations/transfer-type process and the latter is referred to as one-conductor/four-rotations/superimpose-type process.
In the above one-photo-conductor/one-rotation-superimpose-type process, by way of example, four sets of devices are arranged on the side of a belt-shaped or drum-shaped photo conductor. Each set forms a toner image on the photo conductor respectively, Cyan, Magenta, Yellow, or Black. This set has two uniformly charging devices (charging apparatuses) that are a corona charging device, image exposing device (exposing apparatus), and an image developing device (developing apparatus). Unlike in the case of one-photo conductor/four-rotations type process or laterally arranged four photoconductors type process, without transferring the image formed on a photo conductor previously to the recording medium or the intermediate transfer member image forming processes, that is uniformly charging, exposing, and developing, are performed for the image kept on the photo conductor, and then the image of four superimposed colors is formed at the identical position on the photo conductor. Although the two charging devices do not always have to be set up, one of them is used as a neutralization device for removing toner electric potential that has an influence on forming a latent image.
In the above apparatus, ozone natured harmful is generated from the ten corona charging devices combined in the above set of two charging devices, a charging device for the pre-transfer step, and a charging device for the transfer step. So, airflow by fan is provided to force the ozone into a filter that absorbs it.
Heretofore various approaches have been suggested concerning the above image processing apparatus by the one-photo-conductor/one-rotation-superimpose-type process. However, each suggested apparatus includes one charging device or two charging devices per color, as discussed previously. Moreover, a non-contact type charging device, for example corotron or scorotron type corona discharging device, is used so as to avoid disturbing the image formed on the photo conductor before by contacting of the charging device.
In the field of conventional image forming apparatuses, the following documents are known. Japanese Laid Open Publication No. 2003-202752 discloses a developing device that transports toner with a phase-shift electric field for developing, and Japanese Patent No. 3385008 discloses an example of a charging device that charges with a scorotron charging device.
In addition, an image forming apparatus that forms mono-color images by using two different colors individually, but forms full-color images formed superimposing color on color, is known. The apparatus performs the following steps: charging uniformly one time; forming a latent image with three different potential levels by exposing images; developing the intermediate potential pixel as a blank portion, the high potential pixel in normal development method using black toner, for example, and the low potential pixel in reverse development using red toner, for example.
Further, the devices disclosed in Japanese Patent No. 3073126 and Japanese Patent No. 3170901 are known as an image forming apparatus for forming a mono color image using three or four colors individually by more than once exposing the image. And the type of these apparatuses is called one-photo-conductor/one-rotation multi-color type process image forming apparatuses.
Moreover, it is known that a corona charging device, which is used in the one-photo-conductor/one-rotation-superimpose-type process, for example, generates much unnecessary ozone natured harmful with required corona ion. Therefore conventional image forming apparatuses are required to be equipped with a filter for removing ozone. As a result, the control of airflow, using a fan, for example, is also necessary. Consequently, a larger size and a high production cost of the apparatus is a drawback. And there is another problem that is the requirement of maintenance since a user or a maintenance person needs to replace the filter at the end of its lifetime.
On the other hand, contact type charging devices are known for generating little ozone and thus have no need for the control of airflow in the apparatus by fan, for example. However, using contact type charging devices as a device applied to one-photo-conductor/one-rotation-superimpose-type process causes disturbance of the image formed on the conductor previously. Therefore, contact type charging devices can not be in use practically.
As discussed previously, in an image forming apparatus using the one-photo-conductor/one-rotation-superimpose-type process, quantities of the toner developed on the conductor previously are supposed to vary since the photo conductor is charged uniformly from one to three times for forming a latent image of the next color. So a pre-transfer charge is required so as to get quantities of each color toner lined up after forming a full color image using four color toners before transfer. And again, the use of a non-contact type discharging device is required so as to prevent disturbance of the formed image on the conductor. In this regard, the apparatus requires more space and cost, and frequent exchange of the ozone filter because of its lifetime is shortened by the pre-transfer charging with the non-contact type discharging device.
In addition, in a case that the pre-transfer charge is performed as described, allowing that the quantities of toner can be recovered in average amount, it is impossible to reproduce with accuracy including the distribution. Ordinary, the quantities of electrostatic charge is adjusted optimally including the distribution, not only for developing, but also for transferring, so it would be the best to transfer while keeping the original quantities of electrostatic charge including the distribution. However, it is impossible to recover the original quantities. Therefore a problem of losing image quality occurs also.
In the one-photo-conductor/one-rotation/multi-color-type process described above, an apparatus forms toner image on a photo conductor with multiple color toners by exposing and developing repeatedly after charging uniformly once. After that, the apparatus transfers the image onto the recording medium all at once. This process is a method that form images of mono color in different locations depending on each color. In other words, the process cannot superimpose an image made of mono color toner on other image made of other colors in the same location. Originally, the problems to be solved and technical difficulties in the one-conductor/one-rotation-multi-color-type process differs entirely from those in the one-photo-conductor/one-rotation-superimpose-type process. Therefore, the technique of the multi-color type process can not be applied to the technique of the superimpose-type process.